Silicone rubber is frequently used for the extrusion molding of tube, tape, sheet, wire and cable coatings, and the like because silicone rubber offer an excellent heat resistance, cold resistance, weathering resistance, and electrical characteristics. 2,4-Dichlorobenzoyl peroxide, ortho-chlorobenzoyl peroxide, and similar chlorinated benzoyl peroxides have been used as a curing agents for curing silicone rubber compositions used as the feedstock in these extrusion molding applications. However, various problems are associated with silicone rubber compositions that contain these chlorinated benzoyl peroxides: they evolve an unpleasant odor when cured, their cure yields moldings that have a sticky, or tacky surface, and blooming occurs onto the surface of their silicone rubber moldings. Blooming is the phenomenon of whitening of the surface as a result of gradual deposition of decomposition products of the curing agent on the surface of the molded product.
Japanese Patent Application 59-18758 teaches a solution to the above problems by using bis-(ortho-methylbenzoyl) peroxide as a curing agent. Japanese Application 62-185750 teaches a method in which a curing agent is bis-(para-methylbenzoyl) peroxide. However, in the course of curing, silicone rubber compositions which contain the aforementioned methyl-substituted benzoyl peroxides as curing agents, are subject to the formation of voids. This often produces a product formed from such silicone rubber that is has non-uniform physical properties, or causes spark-outs of insulated wires or a decrease in voltage required for insulation breakdown.
It is an object of the present invention to provide a silicone rubber composition which has significantly reduced deviations in the physical properties caused by voids in the products formed from the silicone rubber, as well by significantly reduced spark-outs of insulated wires and little decrease in voltage required for insulation breakdown.